X-ray spectrometry apparatus having a controlled x-ray source

ABSTRACT

SPECTROMETRY APPARATUS CONSISTS OF A VACUUM CHAMBER, AN ELECTRON AND X-RAY EMISSION TUBE, A SAMPLE-HOLDER, A DEVICE PROVIDED WITH SLOTS, AN ANALYSER CRYSTAL AND MEANS FOR MEASURING AN ELECTRICAL VALUE, A VACUUM PUMP AND MEANS FOR INTRODUCING A GAS INSIDE THE TUBE. THE EXTERNAL ENVELOPE OF THE EMISSION TUBE CONTAINS A QUARTZ TUBE PLACED CONCENTRICALLY AND A CATHODE AND AN ANODE BOTH CENTRED BY MEANS OF SAID QUARTZ TUBE, A TUBULAR SOCKET CLOSES THE OPEN END OF THE EXTERNAL ENVELOPE WHILE THE QUARTZ TUBE AND THE SOCKET ARE PROVIDED, ADJACENT OF THE ANODE, WITH LATERAL APERTURES ON THE SAME AXIS, A CAPILLARY DUCT PERMITS THE INTRODUCTION OF A GAS TO THE NEIGHBOURHOOD OF SAID APERTURES, AND SERVO-OPERATING MEANS PERMITS THE CONTROL OF THE FLOW OF GAS BY THE FLUX OF ELECTRONS EMITTED BY THE ANODE.

March 13, 1973 E. LARRIBAU ET Al- X-RAY SPECTROMETRY APPARATUS HAVING ACONTROLLED X-RAY SOURCE 2 Sheets-Sham l Filed Sept. 9. 1970 SQN March13, 1973 E, LARRlBAU ET AL' 3,720,830

- vx--RAY SPECTRUMETRY APPARATUS HAVING A CONTROLLED x-RAY SOURCE Filedsept. o. 1970 l 2 Sheets-smet 2 Fig: 2

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United States Patent O 3,720,830 X-RAY SPECTROMETRY APPARATUS HAVING ACONTROLLED X-RAY SOURCE Etienne Larribau, Lons par Pau, Bernard Grubis,Paris, and .lean Sabores, Burns, France, assignors to Societe Nationaledes Petroles dAquitaine, Paris, France Filed Sept. 9, 1970, Ser. No.70,859 Claims priority, application France, Sept. 12, 1969, 6931060 Int.Cl. G01n 23/22 U.S. Cl. Z50-51.5 6 Claims ABSTRACT F THE DISCLOSURESpectrometry apparatus consists of a vacuum chamber, an electron andX-ray emission tube, a sample-holder, a device provided with slots, ananalyser crystal and means for measuring an electrical value, a vacuumpump and means for introducing a gas inside the tube. The externalenvelope of the emission tube contains a quartz tube placedconcentrically and a cathode and an anode both centred by means of saidquartz tube; a tubular socket closes the open end of the externalenvelope while the quartz tube and the socket are provided, adjacent ofthe anode, with lateral apertures on the same axis; a capillary ductpermits the introduction of a gas to the neighbourhood of saidapertures; and servo-operating means permits the control of the tlow ofgas by the flux of electrons emitted by the anode.

The present invention relates to a spectrometry apparatus and moreparticularly to a tube for the emission of X-rays and electrons, thelatter by back-scattering from the anode.

Crookes X-ray tubes used for the emission of X-rays a filament emittingelectrons by thermo-electric effect. The electrons thus emitted areaccelerated in a linear electrical eld and come to strike a targetwhich, in its turn, emits an X radiation characteristic of the targetwith a constant background which becomes greater as the accelerationvoltage increases. This emission takes place in a high vacuum of about10-5 torr. It is, furthermore known that the ionising effect in gas,described in particular by Gordon Francis in Ionisation Phenomena ofGases, London, 1960, Butterworth edition, in which an electron havingsuticient energy ionises a gas, causes a multiplication of the currentwhich becomes greater as the electric eld increases. The theory of thisphenomenon has been described by MacAiee, in his paper on Phenomena inIonised Gases, presented at the 7th Belgrade Conference in 1965.

It is already known to use in spectrometry the backscattering phenomenonin which an electron striking an anode emits a beam of electrons oflower energy depending both on the nature ofthe anode and on the energyof the incident electrons. As a result of the random variations of theemitted flux of electrons, the electrical current which strikes theanode is not stable.

It was furthermore known to adjust the pressure of the gas in theemission tube by means of adjustable input injections.

The subject of the present invention is a spectrometry apparatuscomprising an emission tube operating in vacuum of from 10-2 to 10-3 mm.Hg having a value comparable to that existing in the spectrometerchamber and permitting the simultaneous use of X-rays and electronsscattered by the anode due to the absence of a window between the wallof the tube and the sample to be analysed.

The special equipment of the tube enables the operator ICC to attain theaims of the invention through a simple and easy-to-build construction.

The spectrometry apparatus according to the invention is more speciallycharacterised in that it is provided with an emission tube, the externaltubular envelope of which contains a quartz tube placed concentricallyand a cathode and an anode both centred by means of said quartz tube; atubular socket closing the open end of the external envelope, the quartztube and the socket being provided in their parts adjacent to the anode,with lateral apertures having the same aixis; a capillary duct for theintroduction of a gas to the neighborhood of said apertures, andservo-operating means for the control of the gas flow by the flux ofelectrons emitted by the anode.

The servo-system can consist of a proportionally adjustable solenoidvalve placed upstream on the capillary duct and which isservo-controlled by one of the electronic parameters of the tube, suchas the current potential, so as to provide the stability of the currentof electrons delivered.

According to one embodiment of the invention, an ammeter connectedbetween the anode and the cathode permits the measurement of the ux ofelectrons emitted by the anode, the measured current controlling aproportionally adjusted solenoid valve placed upstream on said capillaryduct, the servo-control taking place under conditions leading to thestabilisation of the current.

According to another embodiment of the invention, a collector receives apart of the flux of electrons emitted by the anode, and the chargecollected by the collector servo-controlling the solenoid valve permitsthe adjustment of the gaseous flow in the capillary tube.

According to a further embodiment of the invention, a low-energy X-raydetector measures the flux of X-rays emitted by the anode andservo-controls the gaseous ow adjustment solenoid valve.

According to a still further embodiment of the invention, an )(-raydetector receiving the rays emitted by the analysed sample indirectlymeasures the flux of electrons and enables the control of theservo-operating solenoid valve.

The active part of the cold cathode can be made of high purityaluminium, whereas the active part of the anode is made of anappropriate metal, such as tungsten, rhenium, molybdenum, niobium,tantalum or platinum.

The invention will be better understood in the description hereunder ofan embodiment illustrated by the two attached drawings.

FIG. l shows a diagram of the X-ray and electron emission tube.

FIG. 1A shows the target end of the tube connected to a micro-ammeterfor measuring the current between the anode and cathode.

FIG. lB shows the target end of the tube associated with X-rayresponsive means for controlling the gas ow.

. FIG. 2 shows a diagram of an X-ray assembly comprising said X-ray andelectron emission tube.

A hollow cylindrical tube 1 is provided coaxially with a cylindricalmember having a substantially smaller diameter 2, fitted over part ofits length with cooling fins 3. In one end of the tube 1 is a quartztube 4 for electrical the insulation of member 2. The member 2 isprovided with a pellet of pure aluminium 5, screwed, for example, ontothe end of the cylindrical member 2, which is then inserted into thequartz tube. The opposite end of the quartz tube is surrounded by ahollow tubular member 6 which is coaxially attached to the externalhollow cylindrical tube 1. The member 6 is provided with a watercirculation circuit 7. A sleeve 8 made of a refractory material isattached coaxially to the member 2 by connecting means 9. The tightnessbetween this sleeve and part 9 on one hand and part 6 on the other handis performed by a number of seals 10. On the other end of part 6 acopper anode 11 provided, with a target metal pellet 12, inclined at 45on the assembly centre-line, is fixed by means of a bolt.

This pellet 12 faces two apertures 13 pierced in parts 6 .and 13a in thequartz cylinder 4. The common centre-line of these two apertures passesthrough the centre of pellet 12.

On the edge of aperture 13 is adjusted a capillary duct 14 upon which isplaced an adjustment cock or microvalve 15.

The socket 18 and the electrical connecting means 17 are arranged inorder to permit the use of a standard voltage generator whose cable isadapted to feed a tube having a thermionic filament. In contrast to thethermionic filament tube no low voltage supply is necessary in the caseof the tube according to the invention, so that a device comprising anordinary lamp bulb 16 is available to be connected across the lowvoltage terminals of the cable through the conducting strips 16a and16b.

The socket 18, adapted to receive the terminals of a cable used normallyfor a thermionic filament tube, is attached by means of washer 19. Aconnector 17 carried by the socket 18 is provided with passages for lowvoltage conductors which supply through the strips 16a and 16b the lampbulb 16, and for high voltage conductors (not shown) which supply theelectrodes and 11.

An electron collector 20, borne by an insulating rod 20a, receives afraction of the electronic beam emitted by anode 12 when a voltage isapplied to cathode 5.

The high impedance current, collected by 20, is converted into lowimpedance voltage by the electronic assembly 21, having, for example, avibrating condenser. The voltage issued from 21 is applied to acomparator 22, the output current of which controls a motor 23 whichaffects the adjustment of micro-valve 15.

This device operates in the following way:

The electrical field applied between cathode 5 and anode 11, distantfrom each other by two or three centimeters, causes the emission ofelectrodes drawn from the aluminium cathode 5.

Duct 14 feeds the space between the cathode and the anode with a gassuch as argon, nitrogen or even dry air. By discharge at the output ofcapillary duct 14, there is a constant scanning of this space where theexisting pressure is established according to the volume of the annularspace comprised between tube 4 and 8 and the inter-electrode space andthe pressure in the vacuum chamber surrounding the X-ray and electrongenerator tube.

The electrons drawn from cathode 5 meet molecules of gas and ionise thelatter, creating thus a multiplying effect. All the electrons strikeanode 11. Part of the electrons are directly back-scattered, anotherpart penetrates slightly into pellet 12 and is back-scattered with acertain loss of energy, which enlarges the energy spectrum of theelectrons. Another part of the electrons supplies energy to the pellet12, simultaneously causing the emission of photoelectrons and X-rays.

Furthermore, there is heating and partly volatilization of the materialof the pellet.

A part of the beam of electrons is collected by the collector 20.

The high impedance current is converted into low irnpedance voltage bythe vibrating condenser amplifier 21 which delivers a voltage to thecomparator 22 which, by a relay system controls, according to thedifference voltage, a motor 23 having two ways of rotation.

This motor actuates the needle of a micro-valve 15, adjusting the flowof gas in the capillary duct 14.

The beam of electrons emitted is thus stabilized.

In another embodiment of the servo-system the current of electronscirculating between cathode S and anode 12 may be measured by means of amicro-ammeter A (FIG.

4 1A), the output current of which is converted into voltage and appliedto the input of comparator 22.

Similarly, it is possible to measure by means of an X-ray sensitivedevice X (FIG. 1B) the flux of X-rays emitted by a target placed in thepath of the electrons, when this target is provided either with a grid,or a matrix of constant composition.

FIG. 2 shows the assembly of the emitter tube in an X-ray spectrometer.

A vacuum chamber 24 or so-called diffractometry chamber, is connected toa second vacuum chamber 25, socalled emission chamber, in which ismaintained an absolute pressure of about 10-3 torr, obtained by means ofa mercury diffusion pump 27 driven by a vane-pump 28 and connect-ed tothe chamber 25 by said duct 26.

A Pirani gauge 29 is used to measure the absolute pressure. A capillaryduct 30 is connected to an adjustable micro-valve 31 and to an emittertube 32 of the type described in FIG. 1. This tube emits a fiux ofelectrons and soft X-rays which strike a sample 33 energizing by X-fluorescence the heavy elements and emitting the X-ray spectrum of thelight elements. The X-spectra characteristic of the elements whichconstitute the sample, after passing through a collimator such as theSoller slits 34, diffract themselves on a mobile analyser crystal 35rotating around its axis as well as a gaseous flux meter 36goniometrically mounted with a l/2 ratio.

The angle of diffraction is characteristic of the different elements tobe analysed in the sample 33 and the intensity of the radiationscollected enables quantitative analysis.

The detector 37, as well as the sample 33, receives a part of the fiuxof X-rays and electrons and delivers a signal to the integrator 38. Theelectrical signal is delivered to the servo-mechanism 39 with negativecharacteristic, which controls the solenoid valve 31. In particular, ifthe output ux from tube 32 tends to increase, the signal delivered by 37increases, causing an increase in the elec trical value issuing from 38..The negative characteristic servo-mechanism causes the flow ofmicro-valve 31 to decrease and consequently to reduce the output flux oftube 32. inversely, a lowering of the iux produces the reverse eliectand a stabilisation of the output ux of electrons and X-rays from tube32.

What we claim is:

1. In a spectrometry apparatus comprising a vacuum pump and a vacuumchamber, a gas tube in said chamber for the emission of electrons andX-rays, means for introducing into said tube an ionizable gas theionization of which results in a multiplication of the current withinsaid tube, a sample holder in said chamber positioned in the path ofemissions from said tube, Soller slits positioned to collimate theX-rays emitted by the sample, a rotatably mounted analyser crystal fordiffracting the collimated X- rays, and a goniometrical flux meterresponsive to rays diffracted by said crystal, the improvement accordingto which said emission tube contains an anode and a cathode located in aspace in direct environmental communication with said vacuum chamber, acapillary duct connected to introduce into said gas tube a gas fromoutside said tube in the neighborhood of said cathode, gas flow controlmeans in said duct, means to measure an electrical param` eter which ischaracteristic of the operation of said gas tube, and servo meansconnected to said control means and said measuring means to control theflow of gas into said gas tube in response to variations in saidelectrical parameter in either direction from a predetermined value inorder to stabilize the said parameter.

2. An apparatus according to claim 1 wherein the control means comprisesa solenoid valve in the gas duct and said measuring means is responsiveto the current flowing between said anode and cathode.

3. An apparatus according to claim 1 wherein the control means comprisesa solenoid valve in the gas duct and said measuring means is responsiveto the electron emission of the radiation output of the tube.

4. An apparatus according to claim 1 wherein the control means comprisesa solenoid valve in the gas duct and said measuring means is responsiveto the X-ray flux emitted by the sample under analysis.

5. Apparatus as claimed in claim 1 in which said emission tube comprisesa hollow cylindrical external tube (1), an internal cylindrical member(2) inside said external tube, a quartz tube (4) attached to one end ofsaid internal cylindrical member and containing at one end said anodeand cathode, and a tubular hollow member (6) provided with a watercirculation circuit, coaxially attached to said external tube (1) andencircling said quartz tube, said tubular hollow member (6) and saidquartz tube being provided with aligned lateral apertures through whichthe space within said quartz tube in which said anode and cathode arelocated is in environmental communication with said vacuum chamber.

References Cited UNITED STATES PATENTS 3,246,146 4/1966 Cohen et al,Z50-49.5 720,095 2/ 1903 Andrews 250-96 WILLIAM F. LINDQUIST, PrimaryExaminer U.S. Cl. X.R.

Z50-49.5 PE, 96; 313-55; 315-110

